Outer Space Makes Bacteria More Deadly

When we think of the dangers that space flight poses to humans, we tend to worry about macro problems. What if the shuttle’s engine fails? What if an asteroid hits the shuttle? What if the shuttle hits a planet? We do not think of micro problems, like what happens if a microscopic bacterium hitches a ride on the shuttle. But a growing body of space microbiology literature indicates that we should be thinking about such problems. Multiple experiments performed during space shuttle missions suggest bacteria found on Earth thrive in outer space, growing faster, becoming more virulent, and better resisting antibiotics.

In 2006, Atlantis Mission STS-115 astronauts brought Salmonella aboard their shuttle, activating bacterial growth mid-flight. Almost immediately after the shuttle landed on Earth (so that the bacteria could not readjust to earthly conditions), a team of scientists led by Dr. Cheryl Nickerson of Arizona State University infected mice with the extra-terrestrially grown parasites. The results were striking — despite originating from a strain found on Earth, bacteria grown in space were more virulent, killing mice faster, at higher rates, and at lower doses than the same bacteria grown on Earth.

Why does simply growing bacteria in space alter their virulence and other properties? It turns out space changes gene expression in Salmonella. Although the direct cause of this change is still debated, a few hypotheses have been proposed. It may be due to space’s low gravity, which modifies both the way fluid presses against bacterial cells and the chemical composition of this fluid. It’s also possible that low gravity affects the outer membrane composition, changing the amount of material that can enter the bacterial cells.

Other explanations have to do with more direct genetic mechanisms. More efficient exchange of plasmids (circular bacterial DNA) in space could explain genetic changes in some but not all types of bacteria. Finally, excess radiation in space could mutate cellular DNA, resulting in the production of altered proteins.

These theories for altered gene expression might also explain some other curious observations. Bacteria generally grow faster and form larger populations in space. This was first observed on the Russian Mir space station. Over the course of its time in space, Mir became covered in bacterial biofilm, a meshwork of bacteria cells. Since Mir, controlled experiments on multiple space missions have confirmed this increased growth phenomenon. Scientists think bacteria’s ability to move and the amount of available oxygen influence the growth rate in space.

If that wasn’t enough to strike fear of extraterrestrial bacterial infection in your heart, it gets scarier. Bacterial populations grown in space are not only larger and more virulent but also more resistant to antibiotics. In vitro experiments in which bacteria are grown in test tubes aboard space shuttles show that antibiotics are less effective against bacteria grown in space than they are against bacteria grown on Earth. This might have to do with greater resistance of bacteria or decreased drug efficacy and uptake. The prognosis for humans infected in space gets worse, though. Astronauts often show compromised immunity upon returning to Earth from space flight. The reasons behind this observation are not well understood. One hypothesis contends that space’s low gravity alters protein folding in humans, rendering immune system proteins unable to recognize and respond to earthly pathogens.

Studying bacterial growth in space can provide insight into the evolutionary constraints, virulence, and resistance of pathogens on Earth. If we understand what increases bacterial resistance in space, we might be better able to counteract similar resistance mechanisms on Earth. Understanding extraterrestrial bacterial growth is also important for protecting the health of astronauts, who lack access to hospitals during flight. As commercial space flight becomes a more realistic possibility, we need to understand the consequences of infection by these bacteria and of bringing them back to Earth.